The following description will depict a configuration and principle of operation of a conventional optical semiconductor device used in an optical information processing device, which has a function of detecting reproduction signals and various kinds of servo signals, while referring to FIG. 10. A light beam emitted from a semiconductor laser element 101 as a light source is diffracted in a Y direction as viewed in the figure by a three-beam generating diffraction grating element 102, so that a zeroth order light of the same becomes a main beam while the first order lights (±1) become sub beams. These three beams obtained by division are focused on an information recording medium 106 by an objective lens 105 and reflected by the information recording medium 106, and then enter a hologram element 103.
The hologram element 103 is a diffraction grating composed of gratings, each in a curved line form. A reflected light beam from the information recording medium 106 is divided by the hologram element 103, where the +1-order diffracted light 107A is subjected to a converging effect, while the −1-order diffracted light 107B is subjected to a diverging effect, and they are guided to photodetector elements 104A and 104B, respectively. The +1-order diffracted light 107A incident on the photodetector element 104A is focused before a light-receiving surface thereof, whereas the −1-order diffracted light 107B incident on the photodetector element 104B is focused behind a light-receiving surface thereof.
Reproduction signals and focus error signals are detected from a main beam among the reflected light beams having been guided to the photodetector elements 104A and 104B, while tracking error signals are detected from sub beams among the same. Focus servo is performed so that the +1-order diffracted light 107A and the −1-order diffracted light 107B resulting from the division by the hologram element 103 have light spots of substantially the same size on the photodetector elements. Tracking servo is performed so that sub beams have equal quantities of light. The position of the objective lens is controlled by those servos, whereby an appropriate action of the optical semiconductor device as an optical information processing device can be achieved.
In the foregoing conventional optical semiconductor device, the same photodetector elements 104A and 104B are used for detecting reproduction signals and focus error signals. Since the ±1-order diffracted light having been subjected to the converging and diverging effects, respectively, at the hologram element 103 have to be received in a defocused state by the photodetector elements 104A and 104B, the photodetector elements 104A and 104B must have large light-receiving areas, approximately 30000 μm2 each. In the case where the photodetector elements have large light-receiving areas, electric capacitances associated with the photodetector elements increase, thereby impairing the quick responsiveness significantly. This particularly has been a significant problem when CD-ROMs, DVD-ROMs, etc. are reproduced at a high speed, for instance, at several tens of times the normal speed. Furthermore, there has been the following problem as well: in the case where the photodetector element for detecting reproduction signals has a large light-receiving area, stray light components incident thereon (external light, unnecessary reflection) increase, thereby decreasing the signal-to-noise ratio (hereinafter referred to as S/N ratio) of the reproduction signals.